The invention relates to a method of detecting the toner concentration of two component dry developer which is used in the developing of an electrostatic latent image, and an apparatus used to carry out the method.
An electrostatic latent image is converted into a visual image by using two component developer. To this end, toner and carrier are intermixed and rubbed against each other to charge the toner to a polarity which is opposite to the polarity of charge which forms an electrostatic latent image. Coulomb force acting between the toner and carrier causes the toner to be attached to the carrier, and the developer under this condition is applied to a surface to be developed. Carrier usually comprises glass beads, magnetic powder or the like while the toner comprises heat sensitive, pressure sensitive or solvent gas sensitive resin powder containing a color pigment. The purpose of using the carrier in the developer is to prevent a photographic fog in a non-image region by the Coulomb force which pulls the toner portion attaching to non-image regions back to the carrier. In addition, the application of the carrier to the surface to be developed achieves a uniform attachment of a suitable concentration of toner to the surface.
In a known electrophotographic apparatus which utilizes the aforementioned dry developer, it is readily apparent that a varying toner concentration or content of the developer results in a change in the optical density of the image formed on a copy. An electrophotographic process to obtain a plurality of copies by repeating the developing of an electrostatic latent image and the transfer of the developed image onto a transfer sheet is disclosed in U.S. Pat. No. 2,951,443, as well as other patents. To enable the number of copies to be produced without noticeable degradation in the image quality, it is necessary to avoid the erasure of the latent image during the developing step. This can be achieved by increasing the toner concentration where a semi-conductive carrier such as magnetic powder is used. However, an excessively high toner concentration causes a fog to be produced in non-image regions and also causes a reduced flowability of the developer. Thus a control of the toner concentration is required. It is experimentarily established that for a developer including the iron powder carrier having a resistivity on the order of 1.times.10.sup.6 to 1.times.10.sup.8 .OMEGA. cm, a toner concentration in the range from 4 to 15% by weight is satisfactory.
To achieve a proper control of toner concentration, a variety of methods and apparatus have been proposed which detect the toner concentration of a developer contained in a developer vessel to supply toner or developer from time to time in order to maintain a proper value of toner concentration. The detection can be accomplished in two ways. A first technique utilizes the detection of a change in the physical properties of the developer such as color, specific gravity, permeability, dielectric constant, resistance or the like. However, a change in these properties is minimal as compared with a corresponding change in the toner concentration, and hence the resulting signal suffers from a poor S/N ratio. This disadvantageously necessitates a complex and expensive detection arrangement.
Another detection technique employs the optical determination of the toner quantity on a detecting surface which is developed with the developer. The detecting surface may comprise a charged insulating layer or an electrically energized conductive member. An example of the former is a detecting tape which is subjected to corona charging before the developing takes place. The required arrangement is complex and requires an extensive space to permit the deployment of the tape. As a further example, an apparatus is known which regulates the optical density of an image. With this apparatus, a developed image on a photosensitive drum is optically scanned, and a signal indicative of the maximum optical density is used to control the toner concentration. This requires a complicated instrumentation circuit, and it is believed that control parameters must be varied in accordance with an original. Hence, this approach leads to a complex control of the toner concentration. The latter approach is exemplified by U.S. Pat. Nos. 3,376,854, No. 3,430,606, No. 3,526,338, No. 3,527,387, No. 3,604,939, No. 3,635,373, and No. 3,754,821. However, from experiments which will be described later, it is found that when the developing is effected with a voltage applied to the conductive member, the saturation density rises to a substantially high value independently of the toner concentration, rendering it difficult to detect and control the toner concentration with an effective accuracy.
Other examples of the latter approach are given in U.S. Pat. No. 3,777,173 and No. 3,791,744. Essentially, these patents describe a process in which a developer including a glass bead carrier is used to effect a cascade developing of a toner catcher plate, and the concentration of toner trapper by the plate is determined. No positive means is provided which applies an electrical influence upon the catcher plate to trap the toner. This approach is inapplicable to an apparatus which incorporates a magnet brush developing system since it is directed to a cascade developing of the catcher plate.